In the case of recognizing a voice in real environment, there is a problem such that when an utterance voice uttered by the user is influenced by an utterance voice uttered by another person (hereinbelow, called “interfering uttered voice”) and ambient noise, recognition precision deteriorates. As a method of reducing the influence of ambient noise, a method of emphasizing a voice by using a noise canceller and a microphone array is being studied but does not reach complete elimination of noise. One of methods of increasing recognition precision by reducing the influence of ambient noise is a method of suppressing the influence of ambient noise by superimposing weak white noise on an input voice. In the case where ambient noise is weaker than an uttered voice, by whitening the ambient noise with white noise superimposition, the influence can be reduced.
A conventional voice recognizing system using white noise is disclosed in, for example, a patent document 1. The conventional voice recognizing system described in the document has a voice detector for calculating voice-likeness level frame by frame at the time of voice recognition and performs a process of superimposing white noise onto an interval in which voice-likeness level is determined to be low. The conventional voice recognizing system is directed to suppress deterioration in the precision of voice recognition by superimposing noise on a silent interval in voice recognition.
It is known that there is a case that the precision of voice recognition is improved by superimposing weak white noise on an input voice to be recognized.    Patent Document 1: Japanese Unexamined patent publication No. H7-191696.